You're right. The witnesses DO help "wordly"people in some cases. For example, after natural disasters brothers go to devastated areas to help their fellow believers. Afterward they help other people.
However, their reason for helping the others is so that it will make a good witness. True Christians show unconditional love to their fellow humans. The church I now attend - and most others that I know of - for example, feed the homeless on Saturdays, knowing full well these people probably have no intention of joining the church.